1. Technical Field
The present invention relates to a projector, and more particularly to a technique concerning a projector which includes a liquid crystal display as a spatial light modulator.
2. Related Art
Projectors display images by projecting light modulated according to image signals. Conventionally, techniques have been proposed for modulating the light before the light comes into a spatial light modulator of the projector, for example, in addition to modulating the light in the spatial light modulator (see JP-A-2004-354717, for example). When the incoming light to the spatial light modulator is modulated, the projector can display the image in a wider dynamic range than a dynamic range corresponding to the control of the spatial light modulator.
As a spatial light modulator, a liquid crystal display can be employed, for example. To prevent degradation of liquid crystal material called “image persistence”, the polarity of an applied voltage to the liquid crystal display is reversed and the resulting alternate-current voltage with the reversed polarity is applied to drive the liquid crystal display every predetermined time period. When the polarity of the applied voltage is reversed, in other words, when the applied voltage changes its polarity from the positive to the negative, a displayed image sometimes changes its brightness slightly. Even if there is a slight change in the brightness, a viewer recognizes the image in an average brightness as far as a light source unit is constantly on, because in this case the image corresponding to the positive polarity and the image corresponding to the negative polarity are integrated without bias. On the other hand, when the ON-OFF switching of the light source unit is controlled based on Pulse Width Modulation (PWM), for example, the image may be displayed for different time periods corresponding to the positive polarity and to the negative polarity. In this case, since the images of the negative polarity and the positive polarity are not equally integrated, the change in brightness becomes recognizable, causing problems such as gradation shift and non-uniformity of display. Then, high-quality image display is difficult to achieve. As can be seen, techniques as described above have difficulties in displaying high-quality images in a wide dynamic range.